1. Field of the Invention
The present invention relates to EPROM and flash memory cells and, more particularly, EPROM and flash memory cells with source-side injection and a gate dielectric that traps hot electrons during programming.
2. Description of the Related Art
An electrically-programmable read-only-memory (EPROM) cell and a flash memory cell are non-volatile memories that retain data stored in the cell after power to the cell has been removed. EPROM and flash memory cells principally differ from each other in that EPROM cells are erased with ultraviolet (UV) light, while flash cells are electrically erased.
FIG. 1 shows a cross-sectional view that illustrates a prior-art EPROM or flash memory cell 100. As shown in FIG. 1, cell 100 includes spaced-apart n+ source and drain regions 112 and 114 which are formed in a p-type substrate 110, and a channel region 116 which is defined in substrate 110 between source and drain regions 112 and 114.
In addition, cell 100 also includes a layer of gate oxide 120 which is formed over channel region 116, and a floating gate 122 which is formed over gate oxide layer 120. Further, cell 100 additionally includes a layer of interpoly dielectric 124 which is formed over floating gate 122, and a control gate 126 which is formed over dielectric layer 124.
Cell 100 is programmed by applying a programming voltage to control gate 126, a drain voltage to drain region 114, and ground to source region 112. The programming voltage applied to control gate 126 induces a positive potential on floating gate 122 which, in turn, attracts electrons to the surface of channel region 116 to form a channel 130.
In addition, the drain-to-source voltage sets up an electric field which causes electrons to flow from source region 112 to drain region 114 via channel 130. As the electrons flow to drain region 114, the electric field, which has a maximum near drain region 114, accelerates these electrons into having ionizing collisions that form channel hot electrons near drain region 114.
A small percentage of the channel hot electrons are then injected onto floating gate 122 via gate oxide layer 120. Cell 100 is programmed when the number of electrons injected onto floating gate 122 is sufficient to prevent channel 130 from being formed when a read voltage is subsequently applied to control gate 126.
Since electrons are injected onto floating gate 122 near drain region 114, cell 100 is referred to as having drain-side injection. However, by altering the structure of the cell, electron injection can alternately take place near the source region.
When electrons are injected onto a floating gate near the source region, the cell is referred to as having source-side injection. U.S. Pat. No. 5,212,541 to Bergemont discloses a prior-art EPROM cell with source-side injection.
FIG. 2 shows a cross-sectional view that illustrates a source-side injection EPROM cell 200 as disclosed by the '541 patent. FIG. 2 is similar to FIG. 1 and, as a result, utilizes the same reference numerals to designate the structures which are common to both cells.
As shown in FIG. 2, cell 200 differs from cell 100 in that source region 112 no longer lies directly below floating gate 122 and control gate 126, but instead is spaced apart from the region that lies directly below floating and control gates 122 and 126.
Further, cell 200 includes a polysilicon (poly) spacer 210 that is formed over source region 112 and a portion of channel region 116, and is isolated from source region 112, the portion of channel region 116, floating gate 122, and control gate 126.
In operation, cell 200 is programmed in the same manner that cell 100 is programmed except that cell 200 also applies a low positive voltage to poly spacer 210. Under these biasing conditions, the structure of cell 200 alters the drain-to-source electric field so that the electric field has a peak in the channel region that lies below the isolation region that separates poly spacer 210 from floating and control gates 122 and 126.
As a result, channel hot electrons are formed in this channel region where a number of these hot electrons are injected onto floating gate 122. As with cell 100, cell 200 is programmed when the number of electrons injected onto floating gate 122 is sufficient to prevent channel 130 from being formed when a read voltage is subsequently applied to control gate 126.